fix: retire Mixed aggregation and restore native provider paths (2.1.18)

[loLollipop]

Summary

• Retire the local Mixed aggregate platform and its model-family key scopes, vendor billing multipliers and dedicated protocol fallback/stream handling.
• Restore separate Kimi, Zhipu GLM, DeepSeek and MiniMax account/group/key selectors. Keep upstream native OpenAI, Anthropic, Gemini, Grok, Antigravity, Command Code, Cline and TypeSafe paths.
• Preserve native provider profiles and existing protocol conversion, tool history, ordinary model mappings, Composite routing, model allowlists, native group/user pricing, fixed-link shop recharge and upstream update checks.
• Preserve applied database migration files and compatibility columns. Retired Mixed records/non-all key scopes fail closed until explicitly migrated by an operator; do not guess account platforms or modify balances on startup.

Verification

• Frontend full regression: 385 files / 3,058 tests pass; production build and typecheck pass. ESLint found only two stale test imports; removed and the affected file rechecked clean.
• Native HTTP/JSON/SSE tool forwarding, native preauthorization and legacy fail-closed regressions pass. Actual iterator tests cover native single-group compatibility and multi-group permission isolation. Latest-head targeted service, handler, admin and server tests pass.
• Full local backend unit tests pass with the isolated PostgreSQL fixture enabled; latest-head go vet and complete embedded production compilation pass. Exact-head PR CI38790660789 and security38790660622 are entirely successful, including unit/integration, native streaming/tool use, legacy admission, routing/cache/billing concurrency, frontend production build, lint and security.
• Merged main CI38793281389 and tag CI38793337681, plus main/tag security scans, also completed successfully for release commit74a985d3b16d3aa5b54d74dffdc63e91ec0f4bd5.
• Migration: 7 unit tests plus real isolated PostgreSQL apply/check/rollback, late transactional drift and server-side continuation tests pass. Recovery copy-failure and metadata-failure mocks pass.
• Independent source and rollout reviews performed; latest identified multi-group candidate-permission issue is fixed with a concrete regression. No production change has been made at PR preparation time.

Production rollout

The maintainer deployment completed successfully using the formal v2.1.18 GitHub Linux asset, with the release digest, package checksum and embedded commit verified. A validated private full database backup preceded the allowlisted native configuration migration. Existing short aliases became ordinary explicit model mappings; Key identities, pricing configuration and the previous model catalog were retained, without balance conversion or whole-database rollback.

Six bounded owner-admin qwen3.8-27b requests passed across Chat Completions, streaming Responses and streaming Messages, including structured tool arguments, matching IDs, terminal events and nonempty tool-result followups. Synthetic tools were not executed; inference usage was billed normally. Runtime hash/version, ready state and public frontend were verified. This smoke test does not claim all models or all accounts were individually exercised.

Excluded: LinuxDo referral propagation, expanded invitation codes, bespoke reconciliation/journals, new protocol autodetection and the retired Mixed design.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.